Energy Efficient Methods for Isomerization of a C5-C7 Fraction with Dividing Wall Fractional Distillation

ABSTRACT

This invention relates to a method of separating an isomerization zone effluent mixture comprising between 5 and 8 carbon atoms into high octane isomerate product streams and low octane streams which may be recycled to the isomerization zone. The separation process makes use of a dividing wall column to efficiently perform the separation of high octane multibranched paraffins from low octane straight chain and single branched paraffins.

BACKGROUND OF THE INVENTION

This invention relates generally to the isomerization of hydrocarbons.More specifically, the invention involves an isomerization zone and anisomerized products fractionation zone in which a stabilized effluentstream from the isomerization zone is separated into high octane productstreams and low octane product streams by means of fractionaldistillation and by making use of a dividing wall column and anon-divided column. The stabilized isomerization zone effluent isgenerally comprised of hydrocarbons containing between 5 and 8 carbonatoms per molecule.

Isomerization is an important process used in the petroleum industry toincrease the research octane number (RON) of light naphtha feeds. Incurrent practice, the naphtha (C5-C10 fraction) obtained fromatmospheric distillation of petroleum is separated by means offractional distillation into light naphtha (C5-C6 fraction or C5-C7fraction depending on desired volume of light naphtha) and heavy naphtha(C7-C10 fraction or C8-C10 fraction depending on desired volume of lightnaphtha). The light naphtha is generally sent to an isomerizationprocess unit and the heavy naphtha is generally sent to a catalyticreforming process unit. In both the isomerization process unit and thecatalytic reforming process unit, the RON values of the respectivenaphtha fractions are improved. High RON values are a desiredcharacteristic for naphtha streams that are sent to the gasoline poolbecause gasoline spark ignition engines perform better and can achievegreater fuel efficiency with higher RON gasoline.

The product streams from isomerization processes (isomerate), unlike theproduct streams from catalytic reforming processes (reformate) arevirtually free of aromatic compounds. Low aromatic concentrations are adesired characteristic for naphtha streams that are sent to the gasolinepool because of increasingly stringent specifications for aromatics ingasoline. As a result of the increasingly stringent specifications foraromatics in gasoline, there has been growing interest in the petroleumindustry in processing a greater volume of light naphtha inisomerization process units.

The present invention relates in particular to C5-C7 fraction lightnaphtha feeds to isomerization units that are rich in C5-C8 molecules.The C5-C7 fraction is generally produced through fractionation of fullrange naphtha in such a manner that the majority of the C8 moleculesfound in the full range naphtha are excluded from the C5-C7 fraction.However, a small percentage of the C8 molecules from the full rangenaphtha will be included in the C5-C7 fraction as a result of overlapthat is characteristic of distillation processes. Therefore, the term“C5-C7 fraction” will be used herein to designate a fraction thatcontains C5-C8 molecules but in practice is materially a C5-C7 fraction.

Several processes for isomerizing C5-C7 fraction light naphtha feeds aredescribed in the patent literature. Two such examples of recent patentsare U.S. Pat. No. 6,338,791 and U.S. Pat. No. 7,429,685. U.S. Pat. No.6,338,791 describes various process flow schemes to isomerize a C5-C7fraction and separate the isomerization reactor effluent into highoctane streams and low octane streams. U.S. Pat. No. 7,429,685 describesvarious process flow schemes in which the C5-C7 fraction is firstseparated into a C5-C6 fraction and a C7 fraction before passing the twofractions independently to two parallel isomerization reactors, fromwhere the isomerization reactor effluents are separated into high octanestreams and low octane streams. U.S. Pat. No. 7,429,685 describesseveral separation configurations; in one configuration the reactoreffluents are separated independently, and in another configuration thereactor effluents are combined and separated.

The separation of the isomerate reactor effluent in isomerizationprocesses is critical to achieving the desired RON target for theisomerate product. In order to maximize the isomerate product RON, it isdesirable to separate the isomerization reactor effluent into differentmolecular structural classes. In general, multibranched paraffins(paraffins having two or more branches) have higher RON values thanstraight chain and single branched compounds. It is desirable,therefore, to separate the high octane multibranched compounds (as wellas high octane isopentane) as isomerate product and recycle lower octanestraight chain and single branched paraffins to the reactor feed. It isgenerally not desirable to recycle multibranched paraffins to thereactor feed because doing so would result in the conversion of aportion of the high octane multibranched paraffins into lower octanestraight chain and single branched paraffins in the isomerizationreactor.

Several methods that have been utilized to achieve the desiredseparation between high octane components and low octane components inisomerization reactor effluents in applications with C5-C6 fractionlight naphtha feeds are described in Domergue, B., and Watripont, L.World Refining, May 2000, p. 26-30.

None of the methods outlined in the Domergue and Watripont article makeuse of a dividing wall column to separate high octane components and lowoctane components in isomerization reactor effluents. In general, asignificant improvement in the efficiency of separation can be achievedthrough separations that are performed in dividing wall columns comparedwith the use of multiple non-divided columns to perform the sameseparations because of the superior thermal efficiency of dividing wallcolumns. An alternate scheme for achieving the desired separationbetween high octane components and low octane components inisomerization reactor effluents in applications with C5-C6 fractionlight naphtha feeds using a combination of adsorption and a dividingwall column is described in U.S. Pat. No. 6,395,951. Separatingisomerization reactor effluents in applications with C5-C7 fractionlight naphtha feeds is significantly more complicated than inapplications with C5-C6 fraction light naphtha feeds, especially whenhigh values of isomerate RON are required. Schemes that require the useof a deisohexanizer to recover and recycle methylpentane compounds(single branch C6 paraffins) in applications with C5-C6 fraction lightnaphtha feeds increase in complexity in applications with C5-C7 fractionlight naphtha feeds and require the use of a deisohexanizer and adeisoheptanizer to achieve a high isomerate product RON. Deisohexanizercolumns are generally large, costly to construct and install, andconsume large amounts of reboiler energy because of the difficultseparation between close boiling high octane multibranched C6 paraffinssuch as dimethylbutanes and low octane single branched C6 paraffins suchas methylpentanes. Deisoheptanizer columns present the same drawbacks asdeisohexanizer columns because of the difficult separation between closeboiling high octane multibranched C7 paraffins such as dimethylpentanesand low octane single branched C7 paraffins such as methylhexanes.

An example of a conventional method for separating a combinedisomerization zone effluent mixture by fractionation into high octaneand low octane streams is shown in FIG. 1. The charge to theisomerization process is sent via line 12 to charge fractionation zone20. The charge fractionation zone may produce one or more primary feedsto the isomerization zone. Two primary feeds to the isomerization zoneare shown in the example in FIG. 1. The two primary feeds are conductedfrom the charge fractionation zone 20 to isomerization zone 22 via lines14 and 16. In the example shown in FIG. 1, the stream that is conductedvia line 14 represents a C5-C6 fraction and the stream that is conductedvia line 16 represents a C7 fraction. Two recycle streams from theisomerized product fractionation zone are also sent to the isomerizationzone. A C6 rich recycle stream is conducted via line 42 and mixed withthe C5-C6 fraction primary feed to create a combined C5-C6 isomerizationzone feed stream which is conducted via line 18 to isomerization zone22. A C7 rich recycle stream is conducted via line 34 and mixed with theC7 fraction primary feed to create a combined C7 isomerization zone feedstream which is conducted via line 24 to isomerization zone 22. Tworeactor effluent streams exit the isomerization zone via lines 26 and 28and are sent to two independent stabilizers (not shown in FIG. 1) toremove butane and lighter hydrocarbons. A stabilized isomerized productis removed from each of the two stabilizers.

The stabilized isomerized product corresponding to a C5-C6 fraction issent via line 26 to deisohexanizer column 38. The C5-C6 fractionisomerized product is separated into three streams in the deisohexanizercolumn: a first high octane stream comprising the major portion ofhydrocarbons containing 5 carbon atoms and paraffins containing 6 carbonatoms with at least two branches is removed from the first end of thecolumn via line 40, a low octane stream comprising the major portion ofnormal hexane and paraffins containing 6 carbon atoms and a singlebranch is removed as a side stream from an intermediate point in thecolumn via line 42, and a second high octane stream comprising the majorportion of hydrocarbons containing at least 7 carbon atoms is removedfrom the second end of the column via line 44 (note that this stream mayoptionally be recycled to the isomerization zone or to the chargefractionation zone). The term “first end of the column” is used hereinto refer to the overhead system (at the top) of the column and the term“second end of the column” is used herein to refer to the bottom of thecolumn.

The stabilized isomerized product corresponding to a C7 fraction is sentvia line 28 to deisoheptanizer column 30. The C7 fraction isomerizedproduct is separated into three streams in the deisoheptanizer column: afirst high octane stream comprising major portion of hydrocarbonscontaining 6 carbon atoms and paraffins containing 7 carbon atoms withat least two branches is removed from the first end of the column vialine 32, a low octane stream comprising the major portion of normalheptane and paraffins containing 7 carbon atoms and a single branch isremoved as a side stream from an intermediate point in the column vialine 34 and a second high octane stream comprising the major portion ofhydrocarbons containing at least 8 carbon atoms is removed from thesecond end of the column via line 36.

A conventional method for separating isomerization zone effluent streamsas shown in FIG. 1 is energy inefficient because high energy inputs arerequired for each of the two columns to achieve the required separationof high octane and low octane streams. High energy inputs are requiredfor both columns because each of the two columns are designed toseparate close boiling high octane multibranched paraffins and lowoctane single branched paraffins.

The separation scheme presented in U.S. Pat. No. 6,395,951 employs aunique configuration of adsorptive separation followed by fractionaldistillation using a dividing wall column to separate isomerization zoneeffluent streams into high octane and low octane fractions. Low octanestraight chain paraffins such as normal hexane are removed in theabsorptive separation for recycle to the isomerization zone and thedividing wall column separates low octane single branched C6 paraffinsfrom high octane multibranched C6 paraffins and from a high octane C6-C7bottoms stream. The separation in the dividing wall column for thisdesign is notably different than separations which are made in typicaldeisohexanizer column designs in that the majority of (high octane)methylcyclopentane is intentionally removed as part of the high octaneC6-C7 bottoms stream. This contrasts with a typical deisohexanizerdesign that does not have an adsorptive separation section to remove lowoctane straight chain paraffins. Normal hexane (a straight chainmolecule) is present in the feed to a typical deisohexanizer design, andbecause normal hexane has a very low octane value, it is desirable toinclude as much normal hexane as possible in the low octane fractioncontaining low octane paraffins with a single branch so that the normalhexane can be recycled to the isomerization zone for conversion toisomerized products. Normal hexane and methylcyclohexane are closeboiling molecules, and as a result of including the majority of normalhexane in the low octane fraction containing low octane C6 paraffinswith a single branch, the majority of methylcyclopentane is also removedfrom a typical deisohexanizer column in the low octane stream containingnormal hexane and C6 paraffins with a single branch. In effect, themethods described in U.S. Pat. No. 6,395,951 use a dividing wall columnto create high octane and low octane fractions that have differentcompositions with respect to methylcyclopentane than typicaldeisohexanizer separations.

The unique manner in which the method outlined in U.S. Pat. No.6,395,951 using absorptive separation in conjunction with fractionaldistillation means that in order to apply this method to separateisomerization zone effluent streams in applications with C5-C7 fractionlight naphtha feeds, at least two dividing wall columns would berequired; one to segregate low octane single branched C6 paraffins forrecycle to the isomerization zone and a second to segregate low octanesingle branched C7 paraffins for recycle to the isomerization zone. Eachof the two dividing wall columns would require high energy input toachieve the desired separation between close boiling low octane singlebranched paraffins and high octane multibranched paraffins, which makesthe approach described in U.S. Pat. No. 6,395,951 poorly suited forapplications with C5-C7 fraction light naphtha feeds.

The use of a fractional distillation scheme involving a dividing wallcolumn and a non-divided column in the present invention to separate acombined isomerization reactor effluent in a process with a C5-C7fraction light naphtha feed provides significant advantages versusmethods that are currently publically known because the energy intensiveseparations between close boiling high octane multibranched paraffinsand low octane single branched paraffins are combined into a singledividing wall column, thereby reducing the amount of distillation energyinput associated with conventional fractionation techniques.

BRIEF SUMMARY OF THE INVENTION

One purpose of the invention is to separate a combined reactor effluentfrom one or more isomerization reactors into high octane streams and lowoctane streams for the purpose of producing a high octane isomerateproduct and recycling the low octane streams to one or moreisomerization reactors. The combined isomerization reactor effluents aregenerally passed to a stabilizer column which provides a stabilizedisomerized product stream that is removed from the bottom of thestabilizer column. The process which is used to perform the separationof the stabilized isomerized product may create one or more intermediatestreams. The term “intermediate stream” is used herein to describe astream that has not yet been fully separated into high octane and lowoctane fractions and requires further separation to divide the streaminto high octane and low octane fractions. The invention will separatethe stabilized isomerized product stream comprising C5-C8 paraffins withvarying degrees of branching into high octane fraction A comprising themajor portion of hydrocarbons containing 5 carbon atoms and paraffinscontaining 6 carbon atoms with at least two branches, low octanefraction B comprising the major portion of normal hexane and paraffinscontaining 6 carbon atoms and a single branch, high octane fraction Ccomprising the major portion of paraffins containing 7 carbon atoms withat least two branches, low octane fraction D comprising the majorportion of normal heptane and paraffins containing 7 carbon atoms and asingle branch, and high octane fraction E comprising the major portionof hydrocarbons containing at least 8 carbon atoms.

The stabilized isomerized product mixture is separated in a process thatincludes a dividing wall column and a non-divided column. The mixture isintroduced into a dividing wall column which is divided into first andsecond parallel fractionation zones by a dividing wall that extends froma lower end to an upper end within the column, with the first and secondparallel fractionation zones being in open communication at the upperends of each fractionation zone with an upper section of the column thatis undivided and with the first and second parallel fractionation zonesbeing in open communication at the lower ends of each fractionation zonewith a lower section of the column that is undivided.

The stabilized isomerized product mixture is introduced to the dividingwall column at an intermediate point of the first fractionation zone. Anintermediate stream comprising the major portion of normal hexane andparaffins containing 6 carbon atoms and a single branch and the majorportion of hydrocarbons containing 7 carbon atoms with at least twobranches is removed from an intermediate point of the secondfractionation zone of the dividing wall column. The intermediate streamcomprising the major portion of normal hexane and paraffins containing 6carbon atoms and a single branch and the major portion of hydrocarbonscontaining 7 carbon atoms with at least two branches is passed to anon-divided second column. A high octane stream comprising the majorportion of hydrocarbons containing 5 carbon atoms and paraffinscontaining 6 carbon atoms with at least two branches is removed from thefirst end of the dividing wall column and a second high octane streamcomprising the major portion of hydrocarbons containing at least 8carbon atoms is removed from the second end of the dividing wall column.A low octane stream comprising the major portion of normal heptane andparaffins containing 7 carbon atoms and a single branch is removed as aside stream from an intermediate point in the lower undivided section ofthe dividing wall column. A low octane stream comprising the majorportion of normal hexane and paraffins containing 6 carbon atoms and asingle branch is removed from the first end of the non-divided columnand a high octane stream comprising the major portion of hydrocarbonscontaining 7 carbon atoms with at least two branches is removed from thesecond end of the non-divided column.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a simplified process flow diagram that shows aconventional method of parallel fractionation of two isomerization zoneeffluent mixtures using two non-divided columns.

FIG. 2 provides a simplified process flow diagram of a first preferredembodiment of the invention.

FIG. 3 provides a simplified process flow diagram of a second preferredembodiment of the invention.

FIG. 4 provides a simplified process flow diagram of a third preferredembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description is provided herein is exemplary and providesexamples of preferred embodiments of the invention. The description ofthe exemplary embodiments is not intended to limit the use of theinvention to only the exemplary embodiments of the invention describedherein.

The invention is not restricted to any particular type of isomerizationprocess; however, the invention is particularly well suited forisomerization processes which process a light naphtha charge comprisedof a C5-C7 fraction. The invention is also particularly well suited forapplications in which recycle of low octane isomerized products isrequired to meet the RON specification for the isomerate product fromthe overall isomerization process.

The overall isomerization process for processing a C5-C7 light naphthacharge can be generally described as divided into three zones: a chargefractionation zone, where the charge is separated into two or morefractions which may be processed independently in the downstreamisomerization zone; an isomerization zone; and an isomerized productsfractionation zone where the combined reactor effluent is stabilized andthe stabilizer bottoms is separated into high octane isomerized productstreams and low octane recycle streams. The term “overall isomerizationprocess” is used herein to refer to the entirety of the isomerizationprocess. The invention provides an improvement to the processes in theisomerized products fractionation zone.

The isomerization zone may be any form of isomerization zone whichprocesses one or more feed streams containing C5-C7 straight chainhydrocarbons and branched chain hydrocarbons and converts the straightchain hydrocarbons into branched chain hydrocarbons and convertsbranched chain paraffins into paraffins with an increased degree ofbranching. Suitable feeds to the isomerization zone will contain atleast one component from the following group: normal pentane, normalhexane, and normal heptane.

The isomerization zone may be comprised of one or more isomerizationreactor systems as described in U.S. Pat. No. 7,429,685 (note that U.S.Pat. No. 7,429,685 makes reference to two parallel isomerization zonesin contrast to the single isomerization zone with one or more reactorsystems that is described herein). U.S. Pat. No. 7,429,685 describes aprocess in which the isomerization process charge is separated into atleast two fractions (C5-C6 fraction and C7 fraction) for independentprocessing of the two fractions under different isomerization reactorconditions in order to optimize the RON value of the resulting isomerateproducts. Refer to U.S. Pat. No. 7,429,685 for additional informationregarding suitable isomerization catalysts and preferred isomerizationreaction conditions for different feed fractions.

A first exemplary embodiment of the invention is shown in FIG. 2. Thisdrawing is a simplified process flow diagram which does not show detailsfor the process system such as instrumentation and controls, valves,pumps, reboilers, condensers, and heat exchangers. Such details areknown to experienced practitioners of the art.

The charge to the isomerization process is sent via line 102 to chargefractionation zone 20. The charge fractionation zone may produce one ormore primary feeds to the isomerization zone. Two primary feeds to theisomerization zone are shown in FIG. 2. The two primary feeds areconducted from the charge fractionation zone 20 to isomerization zone 22via lines 104 and 106. In this exemplary embodiment of the invention,the stream that is conducted via line 104 represents a C5-C6 fractionand the stream that is conducted via line 106 represents a C7 fraction.Two recycle streams from the isomerized product fractionation zone arealso sent to the isomerization zone. A C6 rich recycle stream isconducted via line 128 and mixed with the C5-C6 fraction primary feed tocreate a combined C5-C6 isomerization zone feed stream which isconducted via line 108 to isomerization zone 22. A C7 rich recyclestream is conducted via line 122 and mixed with the C7 fraction primaryfeed to create a combined C7 isomerization zone feed stream which isconducted via line 110 to isomerization zone 22.

Isomerization zone 22 shown In FIG. 2 illustrates the isomerizationequipment and processes used to efficiently isomerize the isomerizationzone feeds which are conducted via lines 108 and 110 to theisomerization zone. Each of the isomerization zone feeds are isomerizedin isomerization zone 22 in the presence of isomerization catalysts andhydrogen. Isomerization may take place in one or more isomerizationreactor systems, wherein a reactor system may contain one or moreisomerization reactors in series arrangement. Each isomerization reactorsystem may contain different isomerization catalysts and each reactorsystem may operate at different isomerization conditions in order toefficiently isomerize the isomerization zone feeds. In the exemplaryembodiment of the invention shown in FIG. 2, the C5-C6 isomerizationzone feed is intended to be isomerized in an isomerization reactorsystem designed for isomerizing C5-C6 feeds and the C7 isomerizationzone feed is intended to be isomerized in an isomerization reactorsystem designed for isomerizing C7 feeds. The reactor effluent streamsfrom all of the reactor systems are combined into a single combinedisomerization zone effluent in the exemplary embodiment shown in FIG. 2.In the present invention, however, more than one isomerization zoneeffluent may be sent from the isomerization zone to the isomerizedproduct fractionation zone.

The combined effluents from the isomerization reactors which are removedfrom isomerization zone 22 are sent to stabilizer 132 via line 111 toremove butane and lighter hydrocarbons. A stabilized isomerized productis removed from the second end of stabilizer 132 and sent to afractionation system consisting of a dividing wall column and anon-divided column to separate high octane streams from low octanestreams. Butane and lighter hydrocarbons are removed from the first endof stabilizer 132 via line 112. The stabilized isomerized product issent to dividing wall column 114 via line 113. The dividing wall columncontains two parallel fractionation zones which are divided by avertical dividing wall 116. The dividing wall is imperforate andtherefore prevents flow of vapor or liquid from one parallelfractionation zone across the dividing wall to the other parallelfractionation zone. Above the top of each of the two parallelfractionation zones is an upper undivided fractionation zone and belowthe bottom of each of the two parallel fractionation zones is an lowerundivided fractionation zone. Each of the two parallel fractionationzones are in open communication at the top of the parallel fractionationzones with the upper undivided fractionation zone and each of the twoparallel fractionation zones are in open communication at the bottom ofthe parallel fractionation zones with the lower undivided fractionationzone. This arrangement restricts the flow of vapor and liquid fromcrossing from one parallel fractionation zone to another through thedividing wall but allows vapor and liquid to flow around the dividingwall from one parallel fractionation zone to another.

To simplify the discussion of the separation which takes place individing wall column 114, the separation will be discussed in terms ofthe following five fractions which are produced from the isomerizedproduct fractionation zone: fraction A comprising the major portion ofhydrocarbons containing 5 carbon atoms and paraffins containing 6 carbonatoms with at least two branches, which represents the fraction with thelowest boiling point, fraction B comprising the major portion of normalhexane and paraffins containing 6 carbon atoms and a single branch,which represents the fraction with the second lowest boiling point,fraction C comprising the major portion of paraffins containing 7 carbonatoms with at least two branches, which represents the fraction with thethird lowest boiling point, fraction D comprising the major portion ofnormal heptane and paraffins containing 7 carbon atoms and a singlebranch, which represents the fraction with the fourth lowest boilingpoint, and fraction E comprising the major portion of hydrocarbonscontaining at least 8 carbon atoms which represents the fraction withthe highest boiling point.

Fractions A, C, and E are rich in high octane components which makes itadvantageous to use these fractions as constituents of the isomerateproduct that is produced in the overall isomerization process. FractionsB and D are rich in low octane components which can be furtherisomerized to produce high octane components. Therefore it would be moreadvantageous to recycle Fractions B and D to the isomerization zonerather than to use these fractions as constituents of the isomerateproduct that is produced in the overall isomerization process. RecyclingFractions B and D to the isomerization zone increases the octane of thecomposite isomerate product from the overall isomerization process.

The stabilized isomerized product is introduced at an intermediate pointto the feed side, or first parallel fractionation zone, of the dividingwall column. The entirety of Fraction A as well as a portion ofFractions B and C are driven upwards in the first parallel fractionationzone and enter the upper undivided section of the column. In the upperundivided section of the column, Fraction A is driven upwards to the topof the column and the portions of Fractions B and C which were drivenupwards in the first parallel fractionation zone drain down into thesecond parallel fractionation zone. Fraction A is removed via line 118from the first end of the column as a high octane isomerate productstream.

The entirety of Fractions D and E as well as a portion of Fractions Band C drain down through the first parallel fractionation zone and enterthe lower undivided section of the column. The portions of Fractions Band C which drained down through the first parallel fractionation zoneare driven upward into the second parallel fractionation zone.

Within the second parallel fractionation zone, the portions of FractionsB and C which were driven upwards in the first parallel fractionationzone and drained down into the second parallel fractionation zonecombine with the portions of Fractions B and C which drained downthrough the first parallel fractionation zone and were driven upwardinto the second parallel fractionation zone. The entirety of Fractions Band C are removed from an intermediate point in the second parallelfractionation zone via line 120 as a first side draw from the column.

In the lower undivided section of the column, Fraction E drains down tothe bottom of the column and Fraction D drains down to an intermediatepoint in the lower undivided section of the column. Fraction E isremoved via line 124 from the second end of the column as a high octaneisomerate product stream. Fraction D is removed via line 122 from anintermediate point in the lower undivided section of the column as asecond side draw from the column and returned to the isomerization zone.

The mixture containing Fractions B and C that is removed from anintermediate point of the second parallel fractionation zone of thecolumn is sent via line 120 to an intermediate point in non-dividedcolumn 126, where Fraction B is separated from Fraction C. Fraction B isremoved from the first end of the column and returned via line 128 tothe isomerization zone. Fraction C is removed from the second end of thecolumn via line 130 as a high octane isomerate product stream.

The composite high octane isomerate product from the overallisomerization process in the first exemplary embodiment is comprisedfrom the sum of Fractions A, C, and E. Each of these three fractions areremoved from the isomerized product fractionation zone and combined toform the composite isomerate product from the overall isomerizationprocess.

In the first exemplary embodiment of the invention shown in FIG. 2, fourstreams are removed from the dividing wall column. In this embodiment,Fractions D and E are separated in the lower undivided section of thedividing wall column. It is also possible, however, to perform theseparation of Fractions D and E in a second non-divided column byremoving only three streams rather than four from the dividing wallcolumn. In the scenario where Fractions D and E are separated in asecond non-divided column, the entirety of Fractions D and E drain downto the bottom of the dividing wall column. The stream removed from thesecond end of the dividing wall column containing a mixture of FractionsD and E would be sent to a second non-divided column to separateFractions D and E.

A second embodiment of the invention may be used in certain applicationswhere a significant improvement can be made to the composite isomerateproduct RON by recycling C5 molecules in a C5 rich stream that isproduced in the isomerized product fractionation zone back to the chargefractionation zone. In the charge fractionation zone, the C5 rich streamis separated into a high octane isopentane stream which is removed fromthe process as an isomerate product stream, and a low octane normalpentane stream which is sent to the isomerization zone together with theisomerization process charge. The use of a deisopentanizer in a chargefractionation zone to separate isopentane from a feed comprised of theisomerization process charge combined with a C5 recycle stream from theisomerized product fractionation section is well known to experiencedpractitioners of the art.

A simplified process flow diagram of a second exemplary embodiment isshown in FIG. 3. The charge to the isomerization process is sent vialine 202 to charge fractionation zone 20. Also shown entering the chargefractionation zone is a recycle stream from the isomerized productfractionation zone. The charge fractionation zone may produce one ormore primary feeds to the isomerization zone. An isopentane stream isalso removed from the charge fractionation zone via line 230 as a highoctane isomerate product stream. Two primary feeds to the isomerizationzone are shown in FIG. 3. The two primary feeds are conducted from thecharge fractionation zone to the isomerization zone via lines 206 and208. In this exemplary embodiment of the invention, the stream that isconducted via line 206 represents a C5-C6 fraction and the stream thatis conducted via line 208 represents a C7 fraction. Two recycle streamsfrom the isomerized product fractionation zone are also sent to theisomerization zone. A C6 rich recycle stream is conducted via line 226and mixed with the C5-C6 fraction primary feed to create a combinedC5-C6 isomerization zone feed stream which is conducted via line 210 tothe isomerization zone. A C7 rich recycle stream is conducted via line222 and mixed with the C7 fraction primary feed to create a combined C7isomerization zone feed stream which is conducted via line 212 to theisomerization zone.

Isomerization zone 22 shown In FIG. 3 illustrates the isomerizationequipment and processes used to efficiently isomerize the isomerizationzone feeds which are conducted via lines 210 and 212 to theisomerization zone. Each of the isomerization zone feeds are isomerizedin isomerization zone 22 in the presence of isomerization catalysts andhydrogen. Isomerization may take place in one or more isomerizationreactor systems, wherein a reactor system may contain one or moreisomerization reactors in series arrangement. Each isomerization reactorsystem may contain different isomerization catalysts and each reactorsystem may operate at different isomerization conditions in order toefficiently isomerize the isomerization zone feeds. In the exemplaryembodiment of the invention shown in FIG. 3, the C5-C6 isomerizationzone feed is intended to be isomerized in an isomerization reactorsystem designed for isomerizing C5-C6 feeds and the C7 isomerizationzone feed is intended to be isomerized in an isomerization reactorsystem designed for isomerizing C7 feeds. The reactor effluent streamsfrom all of the reactor systems are combined into a single combinedisomerization zone effluent in the exemplary embodiment shown in FIG. 3.In the present invention, however, more than one isomerization zoneeffluent may be sent from the isomerization zone to the isomerizedproduct fractionation zone.

The combined isomerization reactor effluent stream from isomerizationzone 22 is sent to stabilizer 132 via line 213 to remove butane andlighter hydrocarbons. A stabilized isomerized product is removed fromthe second end of stabilizer 132 and sent to a fractionation systemconsisting of a dividing wall column and a non-divided column toseparate high octane streams from low octane streams. Butane and lighterhydrocarbons are removed from the first end of stabilizer 132 via line214. The stabilized isomerized product is sent to dividing wall column114 via line 215.

The stabilized isomerate product is introduced at an intermediate pointto the feed side, or first parallel fractionation zone, of the dividingwall column. The entirety of Fraction A as well as a portion ofFractions B and C are driven upwards in the first parallel fractionationzone and enter the upper undivided section of the column. In the upperundivided section of the column, Fraction A is separated into twosubfractions and the portions of Fractions B and C which were drivenupwards in the first parallel fractionation zone drain down into thesecond parallel fractionation zone. Fraction A is separated in the upperundivided section of the column into a C5 subfraction which is removedvia line 216 from the first end of the column and returned to the chargefractionation zone and a C6 subfraction which is removed via line 218 asa first side draw from the column. The C6 subfraction is removed fromthe process as a high octane isomerate product stream.

The entirety of Fractions D and E as well as a portion of Fractions Band C drain down through the first parallel fractionation zone and enterthe lower undivided section of the column. The portions of Fractions Band C which drained down through the first parallel fractionation zoneare driven upward into the second parallel fractionation zone.

Within the second parallel fractionation zone, the portions of FractionsB and C which were driven upwards in the first parallel fractionationzone and drained down into the second parallel fractionation zonecombine with the portions of Fractions B and C which drained downthrough the first parallel fractionation zone and were driven upwardinto the second parallel fractionation zone. The entirety of Fractions Band C are removed from an intermediate point in the second parallelfractionation zone via line 220 as a second side draw from the column.

In the lower undivided section of the column, Fraction E drains down tothe bottom of the column and Fraction D drains down to an intermediatepoint in the lower undivided section of the column. Fraction E isremoved via line 224 from the second end of the column as a high octaneisomerate product stream. Fraction D is removed via line 222 from anintermediate point in the lower undivided section of the column as athird side draw from the column and returned to the isomerization zone.

The mixture containing Fractions B and C that is removed from anintermediate point of the second parallel fractionation zone of thecolumn is sent to an intermediate point in non-divided column 126, whereFraction B is separated from Fraction C. Fraction B is removed from thefirst end of the column and returned via line 226 to the isomerizationzone. Fraction C is removed from the second end of the column via line228 as a high octane isomerate product stream.

The composite high octane isomerate product from the overallisomerization process in the second exemplary embodiment is comprisedfrom the sum of Fraction C, Fraction E and a portion of Fraction A. Inthis embodiment of the invention, Fraction A is divided into twosubfractions. The first subfraction, a C5 subfraction, is sent from thefirst end of the dividing wall column via line 216 to chargefractionation zone 20, where it is separated into a high octaneisopentane stream that is removed from the charge fractionation zone vialine 230 and a low octane normal pentane stream, which is sent via line206 to isomerization zone 22 together with the primary C5-C6 feed. Thesecond subfraction, a C6 subfraction, is removed via line 218 as a firstside draw from dividing wall column 114 in the isomerized productfractionation zone. The portion of Fraction A that is included in thecomposite high octane isomerate product from the overall isomerizationprocess consists of the isopentane stream that is removed from thecharge fractionation zone via line 230 plus the C6 subfraction that isremoved from the dividing wall column via line 218 in the isomerizedproduct fractionation zone.

In the second exemplary embodiment of the invention shown in FIG. 3,five streams are removed from the dividing wall column. In thisembodiment, Fractions D and E are separated in the lower undividedsection of the dividing wall column. It is also possible, however, toperform the separation of Fractions D and E in a second non-dividedcolumn by removing only four streams rather than five from the dividingwall column. In the scenario where Fractions D and E are separated in asecond non-divided column, the entirety of Fractions D and E drain downto the bottom of the dividing wall column. The stream removed from thesecond end of the dividing wall column containing a mixture of FractionsD and E would be sent to a second non-divided column to separateFractions D and E.

A third embodiment of the invention may be used in certain applicationsin which the designer prefers to keep the isomerized product streamssegregated rather than combining these streams in the isomerizationzone. The isomerized product streams may be intentionally segregated,for example, to reduce the fractionation energy input to the dividingwall column in the isomerized product fractionation zone. It may bepossible to reduce the fractionation energy input by introducing the C7rich stabilized isomerized product stream and the C5-C6 rich stabilizedisomerized product stream to different feed tray locations in thedividing wall column versus a design in which the C7 rich stream and theC5-C6 rich stream are combined and introduced to the dividing wallcolumn at a single feed tray location.

A simplified process flow diagram of a third exemplary embodiment isshown in FIG. 4. The charge to the isomerization process is sent vialine 302 to charge fractionation zone 20. The charge fractionation zonemay produce one or more primary feeds to the isomerization zone. Twoprimary feeds to the isomerization zone are shown in FIG. 4. The twoprimary feeds are conducted from the charge fractionation zone 20 toisomerization zone 22 via lines 304 and 306. In this exemplaryembodiment of the invention, the stream that is conducted via line 304represents a C5-C6 fraction and the stream that is conducted via line306 represents a C7 fraction. Two recycle streams from the isomerizedproduct fractionation zone are also sent to the isomerization zone. A C6rich recycle stream is conducted via line 332 and mixed with the C5-C6fraction primary feed to create a combined C5-C6 isomerization zone feedstream which is conducted via line 308 to isomerization zone 22. A C7rich recycle stream is conducted via line 328 and mixed with the C7fraction primary feed to create a combined C7 isomerization zone feedstream which is conducted via line 310 to isomerization zone 22.

Isomerization zone 22 shown In FIG. 4 illustrates the isomerizationequipment and processes used to efficiently isomerize the isomerizationzone feeds which are conducted via lines 308 and 310 to theisomerization zone. Each of the isomerization zone feeds are isomerizedin isomerization zone 22 in the presence of isomerization catalysts andhydrogen. Isomerization may take place in one or more isomerizationreactor systems, wherein a reactor system may contain one or moreisomerization reactors in series arrangement. Each isomerization reactorsystem may contain different isomerization catalysts and each reactorsystem may operate at different isomerization conditions in order toefficiently isomerize the isomerization zone feeds. In the exemplaryembodiment of the invention shown in FIG. 4, the C5-C6 isomerizationzone feed is intended to be isomerized in an isomerization reactorsystem designed for isomerizing C5-C6 feeds and the C7 isomerizationzone feed is intended to be isomerized in an isomerization reactorsystem designed for isomerizing C7 feeds.

Each of the effluent streams from the isomerization reactors which areremoved from isomerization zone 22 are sent to a stabilizer to removebutane and lighter hydrocarbons. The effluent stream from theisomerization reactor system which isomerizes the C5-C6 reactor feedfraction is removed from isomerization zone 22 and sent to stabilizer132 via line 312 and the effluent stream from the isomerization reactorsystem which isomerizes the C7 reactor feed fraction is removed fromisomerization zone 22 and sent to stabilizer 134 via line 314. Astabilized isomerized product is removed from the second end of eachstabilizer and sent to a fractionation system consisting of a dividingwall column and a non-divided column to separate high octane streamsfrom low octane streams. Butane and lighter hydrocarbons are removedfrom the first end of stabilizer 132 via line 316 and from the first endof stabilizer 134 via line 320. The stabilized isomerized product fromstabilizer 132 is sent to dividing wall column 114 via line 318 and thestabilized isomerized product from stabilizer 134 is sent to dividingwall column 114 via line 322.

The stabilized isomerized product from stabilizer 132 is introduced atan intermediate point to the feed side, or first parallel fractionationzone, of the dividing wall column. The stabilized isomerized productfrom stabilizer 134 may be introduced at an intermediate point to thefeed side, or first parallel fractionation zone, of the dividing wallcolumn, or alternatively may be introduced to an intermediate point inthe undivided section of the dividing wall column which is below thefirst and second parallel fraction zones. The selection of the locationwhere the stabilized isomerized product from stabilizer 134 isintroduced to the dividing wall column will depend on the concentrationof multibranched C5 and C6 molecules in the stabilized isomerizedproduct from stabilizer 134; if the concentration of multibranched C5and C6 molecules in the stabilized isomerized product from stabilizer134 is very low, it may be advantageous to introduce the stabilizedisomerized product from stabilizer 134 at an intermediate point in theundivided section of the dividing wall column which is below the firstand second parallel fraction zones. The entirety of Fraction A as wellas a portion of Fractions B and C are driven upwards in the firstparallel fractionation zone and enter the upper undivided section of thecolumn. In the upper undivided section of the column, Fraction A isdriven upwards to the top of the column and the portions of Fractions Band C which were driven upwards in the first parallel fractionation zonedrain down into the second parallel fractionation zone. Fraction A isremoved via line 324 from the first end of the column as a high octaneisomerate product stream.

The entirety of Fractions D and E as well as a portion of Fractions Band C drain down through the first parallel fractionation zone and enterthe lower undivided section of the column. The portions of Fractions Band C which drained down through the first parallel fractionation zoneare driven upward into the second parallel fractionation zone.

Within the second parallel fractionation zone, the portions of FractionsB and C which were driven upwards in the first parallel fractionationzone and drained down into the second parallel fractionation zonecombine with the portions of Fractions B and C which drained downthrough the first parallel fractionation zone and were driven upwardinto the second parallel fractionation zone. The entirety of Fractions Band C are removed from an intermediate point in the second parallelfractionation zone via line 326 as a first side draw from the column.

In the lower undivided section of the column, Fraction E drains down tothe bottom of the column and Fraction D drains down to an intermediatepoint in the lower undivided section of the column. Fraction E isremoved via line 330 from the second end of the column as a high octaneisomerate product stream. Fraction D is removed via line 328 from anintermediate point in the lower undivided section of the column as asecond side draw from the column and returned to the isomerization zone.

The mixture containing Fractions B and C that is removed from anintermediate point of the second parallel fractionation zone of thecolumn is sent via line 326 to an intermediate point in non-dividedcolumn 126, where Fraction B is separated from Fraction C. Fraction B isremoved from the first end of the column and returned via line 332 tothe isomerization zone. Fraction C is removed from the second end of thecolumn via line 334 as a high octane isomerate product stream.

The composite high octane isomerate product from the overallisomerization process in the first exemplary embodiment is comprisedfrom the sum of Fractions A, C, and E. Each of these three fractions areremoved from the isomerized product fractionation zone and combined toform the composite isomerate product from the overall isomerizationprocess.

In the third exemplary embodiment of the invention shown in FIG. 4, fourstreams are removed from the dividing wall column. In this embodiment,Fractions D and E are separated in the lower undivided section of thedividing wall column. It is also possible, however, to perform theseparation of Fractions D and E in a second non-divided column byremoving only three streams rather than four from the dividing wallcolumn. In the scenario where Fractions D and E are separated in asecond non-divided column, the entirety of Fractions D and E drain downto the bottom of the dividing wall column. The stream removed from thesecond end of the dividing wall column containing a mixture of FractionsD and E would be sent to a second non-divided column to separateFractions D and E.

The claimed invention is:
 1. An isomerization process having anisomerized product fractionation zone, said process comprising:contacting, in an isomerization zone, one or more feeds, wherein eachfeed contains at least one component from the group consisting of normalpentane, normal hexane, and normal heptane in one or more isomerizationreactors, wherein each isomerization reactor may contain differentisomerization catalysts and each reactor may operate at differentisomerization conditions, to convert at least a portion of the normalpentane, normal hexane, and normal heptane which may be found in thefeeds into isomerized products and form one or more isomerizationreactor effluent streams which are combined into a single isomerizationzone effluent containing at least normal pentane, normal hexane, normalheptane and isomerized products; passing the isomerization zone effluentinto an isomerized product fractionation zone comprising a stabilizer,and a dividing wall column, wherein the isomerization zone effluent ispassed into a stabilizer and a stabilized isomerized product is removedfrom the second end of the stabilizer; passing the stabilized isomerizedproduct into a dividing wall column divided into at least a first andsecond parallel fractionation zones by a dividing wall, with the firstand second fractionation zones each having an upper end and a lower endlocated within the dividing wall column, wherein the first and secondparallel fractionation zones are in open communication at the upper endswith an undivided upper section of the column and wherein the first andsecond parallel fractionation zones are in open communication at thelower ends with an undivided lower section of the column, and whereinthe stabilized isomerized product enters the column at an intermediatepoint in the first parallel fractionation zone; removing an intermediatestream comprising the major portion of normal hexane, the major portionof paraffins containing 6 carbon atoms and a single branch, and themajor portion of paraffins containing 7 carbon atoms with at least twobranches as a side stream from an intermediate point of the secondparallel fractionation zone of the dividing wall column; and removing atleast three streams from the dividing wall column wherein each of theremoved streams can be considered as high octane streams or low octanestreams or alternatively may be considered intermediate streams whichare further separated to produce high octane or low octane streams. 2.The process according to claim 1, wherein said intermediate stream ispassed from an intermediate point of the second parallel fractionationzone of the dividing wall column into a non-divided column.
 3. Theprocess according to claim 2, wherein a low octane stream comprising themajor portion of normal hexane and paraffins containing 6 carbon atomsand a single branch is removed from the first end of the non-dividedcolumn; and wherein a high octane stream comprising the major portion ofparaffins containing 7 carbon atoms with at least two branches isremoved from the second end of the non-divided column.
 4. The processaccording to claim 1, wherein a high octane stream comprising the majorportion of hydrocarbons containing 5 carbon atoms and paraffinscontaining 6 carbon atoms with at least two branches is removed from thefirst end of the dividing wall column.
 5. The process according to claim1, wherein a high octane stream comprising the major portion ofhydrocarbons containing at least 8 carbon atoms is removed from thesecond end of the dividing wall column.
 6. The process according toclaim 1, wherein a low octane stream comprising the major portion ofnormal heptane and paraffins containing 7 carbon atoms and a singlebranch is removed as a side stream from an intermediate point in theundivided section of the dividing wall column which is below the firstand second parallel fractionation zones.
 7. The process according toclaim 1, wherein a second intermediate stream comprising the majorportion of hydrocarbons containing 5 carbon atoms is removed from thefirst end of the dividing wall column and passed to a chargefractionation zone, wherein the majority of isopentane is removed fromsaid second intermediate stream and recovered as a high octane stream inthe charge fractionation zone.
 8. The process according to claim 7,wherein a high octane stream comprising the major portion of paraffinscontaining 6 carbon atoms with at least two branches is removed as aside stream from an intermediate point in the undivided section of thedividing wall column which is above the first and second parallelfractionation zones.
 9. An isomerization process having an isomerizedproduct fractionation zone, said process comprising: contacting, in anisomerization zone, one or more feeds, wherein each feed contains atleast one component from the group consisting of normal pentane, normalhexane, and normal heptane in one or more isomerization reactors,wherein each isomerization reactor may contain different isomerizationcatalysts and each reactor may operate at different isomerizationconditions, to convert at least a portion of the normal pentane, normalhexane, and normal heptane which may be found in the feeds intoisomerized products and form one or more isomerization reactor effluentstreams; passing each of said isomerization reactor effluent streamsinto an isomerized product fractionation zone comprising one or morestabilizers and a dividing wall column, wherein each of theisomerization reactor effluent streams is passed into a stabilizerwithout combining isomerization reactor effluent streams, and wherein astabilized isomerized product stream is removed from the second end ofeach of the stabilizers; passing each of the said stabilized isomerizedproduct streams into a dividing wall column divided into at least afirst and second parallel fractionation zones by a dividing wall, withthe first and second fractionation zones each having an upper end and alower end located within the dividing wall column, wherein the first andsecond parallel fractionation zones are in open communication at theupper ends with an undivided upper section of the column and wherein thefirst and second parallel fractionation zones are in open communicationat the lower ends with an undivided lower section of the column, andwherein each of the stabilized isomerized product streams enters thecolumn at an intermediate point in the first parallel fractionation zoneor alternatively enters the column at an intermediate point in theundivided section of the column which is below the first and secondparallel fractionation zones; removing an intermediate stream comprisingthe major portion of normal hexane, the major portion of paraffinscontaining 6 carbon atoms and a single branch, and the major portion ofparaffins containing 7 carbon atoms with at least two branches as a sidestream from an intermediate point of the second parallel fractionationzone of the dividing wall column; and removing at least three streamsfrom the dividing wall column wherein each of the removed streams can beconsidered as high octane streams or low octane streams or alternativelymay be considered intermediate streams which are further separated toproduce high octane or low octane streams.
 10. The process according toclaim 9, wherein said intermediate stream is passed from an intermediatepoint of the second parallel fractionation zone of the dividing wallcolumn into a non-divided column.
 11. The process according to claim 10,wherein a low octane stream comprising the major portion of normalhexane and paraffins containing 6 carbon atoms and a single branch isremoved from the first end of the non-divided column; and wherein a highoctane stream comprising the major portion of paraffins containing 7carbon atoms with at least two branches is removed from the second endof the non-divided column.
 12. The process according to claim 9, whereina high octane stream comprising the major portion of hydrocarbonscontaining 5 carbon atoms and paraffins containing 6 carbon atoms withat least two branches is removed from the first end of the dividing wallcolumn.
 13. The process according to claim 9, wherein a high octanestream comprising the major portion of hydrocarbons containing at least8 carbon atoms is removed from the second end of the dividing wallcolumn.
 14. The process according to claim 9, wherein a low octanestream comprising the major portion of normal heptane and paraffinscontaining 7 carbon atoms and a single branch is removed as a sidestream from an intermediate point in the undivided section of thedividing wall column which is below the first and second parallelfractionation zones.
 15. The process according to claim 9, wherein asecond intermediate stream comprising the major portion of hydrocarbonscontaining 5 carbon atoms is removed from the first end of the dividingwall column and passed to a charge fractionation zone, wherein themajority of isopentane is removed from said second intermediate streamand recovered as a high octane stream in the charge fractionation zone.16. The process according to claim 15, wherein a high octane streamcomprising the major portion of paraffins containing 6 carbon atoms withat least two branches is removed as a side stream from an intermediatepoint in the undivided section of the dividing wall column which isabove the first and second parallel fractionation zones.